Proppants are solids such as sand, ceramic, polymer, or composite particles, that are often used during fracture stimulation to keep a fracture open by resisting the closure stress applied by the geological formation above the fracture.
In many situations, a substantial portion of the proppant does not remain in a fracture where it has been placed but instead flows back to the wellbore, so that it is valuable to be able to assess the extent of any flowback. Furthermore, a knowledge of the locations of the proppant particles can also provide valuable information about the fracture geometry. The ability to monitor the locations of the proppant particles over time after their placement in a downhole environment is, therefore, a highly desirable objective. Progress towards the attainment of this objective has hitherto been both difficult to make and limited in its scope.
U.S. patent application Ser. No. 12/206,867 teaches a method for “tagging” proppants based on the use of new composite proppant compositions containing dispersed fillers whose electromagnetic properties change under a mechanical stress such as the closure stress of a fracture, and is incorporated in its entirety herein by reference.
Several additional publications will be cited and discussed briefly in the paragraphs that follow. We emphasize that we do not consider any of these publications to constitute prior art for our invention, and that they are being cited and discussed as general background information.
The patent application publication to Huang (U.S. 20080139419), assigned to Baker Hughes Incorporated, provides for “Viscosity Enhancers for Viscoelastic Surfactant Stimulaton Fluids”. Discussed is the addition of pyroelectric crystal and/or piezoelectric crystal particles to an aqueous viscoelastic surfactant (VES) fluid to demonstrate improved, enhanced or increased viscosity of the VES fluid. The viscosity enhancers herein are believed to be particularly useful in VES-gelled fluids used for well completion or stimulation and other uses and applications where the viscosity of VES-gelled aqueous fluids may be increased. The VES-gelled fluids may further comprise proppants or gravel, if they are intended for use as fracturing fluids or gravel packing fluids, respectively; although such uses do not require that the fluids include proppants or gravel.
The patent application publication to Marya et al. (U.S. 20080149345), assigned to Schlumberger Technology Corporation, provides for “Smart Actuation Materials Triggered by Degradation in Oilfield Environments and Methods of Use”. Disclosed is a material placed in a downhole drilling environment that is responsive electrically or magnetically to said environment. This material can be a proppant.
The patent application publication to Fripp (U.S. 20070131424), assigned to Halliburton Energy Services, provides for “Proppant for Use in a Subterranean Formation”. Disclosed is a proppant composition that can include a layer of material able to respond to pressures within the drilling environment. The disclosure states that this can be either an electrically responsive or a magnetically responsive substance.
The patent application publication to Funk et al. (U.S. 20080062036), assigned to Hexion Specialty Chemicals, provides for “Logging Device with Down-Hole Transceiver for Operation in Extreme Temperatures”. Disclosed is a method for measuring the geometry of a propped fracture in a subterranean environment. Proppants having electrical conductivity are discussed wherein said proppants consist of coated thermoset polymer particles. The coating can have piezoelectric properties. The disclosure does not appear to mention mechanical stress as being useful for any embodiment of the invention that it teaches.
The patent application publication to Rediger et al. (U.S. 20080283243), assigned to Georgia-Pacific Chemicals, provides approaches for “Reducing Flow-back in Well Treating Materials”. It teaches the placement of magnetic coatings on proppant particles to stabilize a proppant pack and thus reduce particulate flowback and fines transport. The magnetic particles are applied in a powdered form. They may be adhered to a proppant substrate by using various methods. Preferred methods include the use of (a) a hot melt (thermoplastic) adhesive (possibly comprising a thermoplastic resin and/or a wax powder), and (b) a phenol-formaldehyde novolac resin crosslinked with a hexamine (resulting in a thermoset adhesive after crosslinking).
The patent publication to Ellingsen (U.S. Pat. No. 6,499,536), assigned to Eureka Oil ASA, provides for a “Method to Increase the Oil Production from an Oil Reservoir”. A magnetic or magnetostrictive material is injected through an oil well into the oil reservoir and then the material is vibrated with the aid of an alternating electric field. Oil is then drawn from the same reservoir from the same well in which the magnetic or magnetostrictive material was injected. The vibrations created in the injected material can be changed by changing the frequency of the applied electric current passed into the reservoir.
The following two books provide general background information on piezoelectric and/or magnetostrictive materials: APC International, Ltd., “Piezoelectric Ceramics: Principles and Applications” (2002); and G. Engdahl (editor), “Handbook of Giant Magnetostrictive Materials,” Academic Press, New York (2000).